Metal cyanines, such as phthalocyanines and naphthalocyanines, are excellent candidates for IR absorbing dyes. IR absorbing dyes have numerous applications, such as optical recording systems, thermal writing displays, laser filters, infrared photography, medical applications and printing. Typically, it is desirable for the dyes used in these applications to have strong absorption in the near-IR at the emission wavelengths of semiconductor lasers (e.g. between about 700 and 2000 nm, preferably between about 700 and 1000 nm). In optical recording technology, for example, gallium aluminium arsenide (GaAlAs) and indium phosphide (InP) diode lasers are widely used as light sources.
Another important application of IR dyes is in inks, such as printing inks. The storage and retrieval of digital information in printed form is particularly important. A familiar example of this technology is the use of printed, scannable bar codes. Bar codes are typically printed onto tags or labels associated with a particular product and contain information about the product, such as its identity, price etc. Bar codes are usually printed in lines of visible black ink, and detected using visible light from a scanner. The scanner typically comprises an LED or laser (e.g. a HeNe laser, which emits light at 633 nm) light source and a photocell for detecting reflected light. Black dyes suitable for use in barcode inks are described in, for example, WO03/074613.
However, in other applications of this technology (e.g. security tagging) it is desirable to have a barcode, or other intelligible marking, printed with an ink that is invisible to the unaided eye, but which can be detected under UV or IR light.
An especially important application of detectable invisible ink is in automatic identification systems, and especially “netpage” and “Hyperlabel™” systems. Netpage systems are described in the following patent applications, all of which are incorporated herein by reference.